The present invention relates generally to vapor generation and recovery systems and in particular, to vapor degreasing and phosphatizing apparatus.
Metal cleaning and phosphatizing are manufacturing processes often used by industry today. Metal cleaning is usually done by a vapor degreasing apparatus which generally comprises a tank or compartment having a reservoir for boiling solvent and a region for maintaining a vapor zone above the solvent reservoir. The vapor zone is controlled by cooling coils disposed a predetermined distance above the level of solvent, which condense the rising vapor and return it to the reservoir. Essentially, an equilibrium is established between the vaporizing and condensing solvent so that a closed system is established in which solvent escape from the apparatus is minimized.
In use, an article to be cleaned is either immersed in the boiling bath or merely passed through the vapor zone to effect cleaning. Alternatively, the article may be sprayed with liquid solvent while in the vapor zone. In both cases, as the article is lifted from the vapor zone, any solvent condensed on the surfaces of the article is evaporated. The article is then both clean and dry upon removal from the vapor degreasing apparatus.
Non aqueous, solvent based metal phosphatizing, although a process distinct from vapor degreasing, utilizes a very similar apparatus. Like metal degreasing, it requires a bath of boiling solvent or phosphatizing solution and the maintenance of a vapor zone above the reservoir. Articles to be treated are either immersed in the solvent directly or in the vapor zone for a predetermined time. As articles are removed from the apparatus, the solvent evaporates, leaving behind a protective phosphate residue or deposit. In both processes, the apparatus used must include a source of heat for the solvent reservoir and a source of cooling for the vapor condenser. In many prior art vapor degreasers, the solvent reservoir is heated by steam or electric heaters. The condenser is cooled by circulating cold water through the condenser coils. In most, if not all of these vapor degreasers, the hot and cold sources for the solvent bath and condenser, respectively, are not interrelated and both involve non-recoverable energy use.
In more recent vapor degreasers, it has been recognized that it would be advantageous to utilize the heat energy absorbed by the condenser for heating the solvent. It has been suggested that a heat pump be employed to remove heat from the vapor condenser and convey it to the solvent reservoir. Devices have been constructed embodying this concept which utilize a heat-emitting section of the heat pump (more commonly called a refrigerant condenser) to heat the solvent reservoir, and a heat absorbing section of the heat pump system (commonly called an evaporator) to condense the solvent vapor and define the vapor zone. Structurally, the vapor degreaser would include a conventional tank or compartment defining, in part, a solvent reservoir. A heat exchanger forming the heat-emitting portion of the heat pump system would be disposed in the solvent reservoir. High pressure refrigerant circulated and subsequently condensed in the heat exchanger would heat the solvent.
The vapor condenser, on the other hand, being in actuality a refrigerant evaporator, would receive liquid refrigerant from the heat pump system through an expansion valve and evaporate it to provide cooling for the vapor condenser. The evaporating refrigerant would absorb heat from the condensing vapor, and upon re-compression by the heat pump compressor, would release this heat in the heat-emitting section of the heat pump system. It should be apparent that a heat pump associated with the vapor degreaser could provide an energy efficient vapor degreasing system.
Commercial vapor degreasers utilizing a heat pump system for a source of energy have not been entirely satisfactory. In many systems, excessive system startup time at the commencement of a work day has been experienced. For the heat pump system to operate efficiently, the heat absorbed by the vapor condenser should substantially equal the heat released by the heat emitting section. When the vapor degreaser is first turned on, little or no solvent vapor is available from which the vapor condenser can absorb heat, resulting in incomplete refrigerant evaporation in the vapor condenser. As a result, less heat is available for transfer to the heat-emitting section of the heat pump system; and thus, the time needed to bring the solvent to the boiling point can be excessively long. Moreover, a substantial thermal load imbalance across the heat pump may occur during this startup phase, resulting in compressor cycling or shutdown. To obviate this startup problem, it has been suggested that auxiliary electric heaters be placed in the solvent bath to initially heat the solvent at system startup. This increases energy consumption.
A second problem associated with heat pump vapor degreasers is the thermal imbalance caused by the heat load placed on the system by the articles being treated. It has been found that if the temperature of the articles being treated varies, system stability is adversely affected. This often occures where articles are stored both indoors, at ambient temperatures, and outdoors at substantially lower temperatures. It was found that the vapor degreaser could not accommodate the abrupt change in temperature caused by the articles being treated.
A third problem is related to the type of solvent used. One of the most common solvents in use today is trichloroethylene having a boiling point of approximately 188.degree. F. The rather high reservoir temperature necessary to boil this solvent requires that the heat emitting section of the heat pump be operated well above the boiling temperature of the solvent. Obtaining temperatures in excess of 200.degree. F. from a heat pump system requires rather sophisticated and clostly refrigeration equipment. Furthermore, as the difference in the vaporizing and condensing temperatures for the degreasing solvent increases, the work efficiency of the heat pump decreases. For this reason, methylene chloride, having a boiling point lower than 120.degree. F., has been suggested as an alternate solvent for vapor degreasers employing the heat pump as a source of heating and cooling. The relatively low boiling point of methylene chloride requires a well balanced vapor degreasing system to minimize solvent escape, especially in high temperature environments often encountered in manufacturing plants. Even minor thermal imbalances must be avoided to avoid substantial solvent loss from the degreasing compartment. Prior art vapor degreasers utilizing direct heat exchange between the respective heat pump sections and the solvent reservoir and vapor condenser could not provide the necessary system stability, particularly in larger degreasing units.
Fourthly, although methylene chloride like other halogenated solvents when first introduced into the reservoir is chemically neutral, during use it often takes on corrosive properties and can attack the hardware and plumbing of the degreaser. In systems where the heat pump refrigerant is circulated through a heat exchanger immersed in the solvent reservoir, failure of the plumbing or heat exchanger could result in the release of toxic refrigerant into the atmosphere. Thus, known degreasers using a heat pump as a source of heating and cooling have not been totally acceptable when methylene chloride is used as a degreasing solvent.
The same considerations apply to the metal phosphatizing process, for methylene chloride is often used as a constituent of the phosphatizing bath. Thermal equilibrium in the apparatus as well as solvent evaporation and plumbing corrosion are all factors in the process.